Conventionally, in forming transmission lines intersecting with each other on a monolithic microwave integrated circuit (hereinafter referred to as “MMIC”) having a GaAsFET as an active element, there has been widely used an air bridge wiring structure using air as an interlayer insulating material in order to reduce parasitic capacitance between wirings.
However, such a structure has the problem that in implementing layout with long wiring, a contact occurs between transmission lines or an air bridge wiring itself breaks, which results from an air bridge crushed because of static electricity, humidity, attraction of gravity, temperature, physical impact or dead weight of air bridge wiring itself. And hence MMIC may malfunction.
Also, if a metal film thickness of wiring is increased according to an air bridge wiring distance in order to avoid such a problem, manufacturing processes or labor time increase and hence loading on manufacturing machines and material costs increase, which causes a problem with manufacturing costs.
In view of the above problems, Japanese Patent Application Laid-Open No. 05-152452 has proposed a method to make the length of air bridge wiring short virtually by providing support pillars at regular intervals along the air bridge wiring in order to compensate for a long air bridge wiring length without need of changing a metal film thickness. However, this method has a following problem that the support pillars themselves become capacitor components resulting in excess floating capacitance, which is not preferable in design.
As a method for increasing mechanical strength of air bridge wiring itself, a method for forming a deflection-reinforcing member on the top of air bridge wiring has been disclosed in Japanese Patent Application Laid-Open No. 06-089940. In addition, a method for forming a sectional shape of an air bridge wiring into T, II or H shape has been disclosed in Japanese Patent Application Laid-Open No. 05-218213. Further, a method for making widths of both ends of an air bridge wiring wider than that of the center portion of the air bridge wiring has been disclosed in Japanese Patent Application Laid-Open No. 07-135251.
However, such methods have a problem that manufacturing processes, labor time, loading on manufacturing machines and material costs increase, which causes a problem with manufacturing costs.